Vibration isolator with displacement structure

ABSTRACT

A vibration isolator is provided comprising a base structure, a load structure, a displacement structure and at least one vertical air gap formed by opposing and substantially parallel walls of the base structure and the load structure. The opposing walls being at least partly covered by respective arrays of permanent magnets, neighboring magnets in the arrays having alternating magnetization directions, an arrangement of the permanent magnets in the arrays being such that a gravitational force on the load structure is substantially compensated by a net magnetic force of the base structure on the load structure. The displacement structure relatively displaces arrays of permanent magnets of the opposing walls with respect to each along the air gap for adjusting a load capacity of the vibration isolator.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a Continuation-in-Part of application Ser. No. 13/977,972, filed 2 Jul. 2013, which is the U.S. national phase of International Application No. PCT/EP2011/073785, filed 22 Dec. 2011, which designated the U.S. and claims priority to EP Application No. 11150034.4, filed 3 Jan. 2011, the entire contents of each of which are hereby incorporated by reference.

FIELD OF THE INVENTION

This invention relates to a vibration isolator comprising a base structure, a load structure and means for compensating a gravitational force on the load structure.

BACKGROUND OF THE INVENTION

Many industrial engineers, such as in lithographic industry, electron beam microscopy and space applications, deal with accurate positioning systems. Vibrations and other types of mechanical disturbances in such machines easily show up as a major factor in limiting the achievable accuracy, hence require significant reduction. Precise reproduction of the features that continue to get smaller requires good isolation from the environment while productivity concerns driven by market requirements require faster motion. Such demands impose special constraints on a vibration isolation design. Since, in many cases, the structural design of the isolated mass provides little inherent vibration isolation, and passive means provide insufficient isolation over the full required bandwidth, active means are often utilized to provide vibration control. In such applications, high-precision vibration isolation of a large payload with a high mass often requires vacuum compatibility, a contactless structure, high force density and low stiffness.

Air-based solutions are commonly used for actively isolating and controlling vibrations and other types of mechanical disturbances. In most lithographic applications, so-called air mounts are used, which are supplemented by electromechanical Lorentz-actuators providing stability control. A control valve regulates the flow of compressed air into a large air tank acting as a pneumatic spring. Unlike steel coil springs, the resonant frequency of this system is nearly independent of the mass of the payload, and the height control valve regulates the operating height. This provides gravity compensation and spring stiffness, where Lorentz actuators ensure stability and accurate positioning in all degrees of freedom.

The isolation bandwidth of the currently often used pneumatic isolators is generally limited. As a result the vibrations at elevated frequencies are not properly extinguished, which limits the performance of the machine being isolated. Furthermore, air bearings are only suitable for vacuum conditions if significant structural changes are applied which may adversely affect their performance.

Magnet-based vibration isolation systems are increasingly considered to be a feasible alternative for the passive or pneumatic vibration isolation systems. They offer distinct features such as being clean, noiseless and vibration and maintenance free. For these reasons they are increasingly being considered for use in vibration isolation applications. Examples of magnet-based vibration isolators are, e.g., found in U.S. Pat. No. 6,307,285

In ‘Zero-stiffness magnetic springs for active vibration isolation’ by Robertson et al., a permanent magnet system is used for obtaining a low stiffness vibration isolation system. This contactless magnetic spring uses attracting magnetic forces from the magnet above the load structure (negative spring, vertically unstable) and repelling magnetic forces from the bottom side (positive spring, vertically stable). These magnetic forces are oriented mainly along the axis of magnetization of the permanent magnets. The resulting vertical magnetic force compensates for the gravity force which is pulling down the load structure. Because the negative spring on the top and the positive spring at the bottom are placed in a parallel configuration, their respective spring stiffness add up and the resulting stiffness is near zero. This gives a vibration isolation system with a low natural resonance frequency to provide enhanced vibration isolation. It is, however, a disadvantage of this magnetic spring type that the low spring stiffness is very local and as a result the spring stiffness varies significantly with the position. This allows for small movements only when low and constant spring stiffness is required or it requires significant control efforts for stabilization and isolation over a larger movement range. Furthermore, such a double-sided topology requires a ‘sandwiched’ construction which may proof to be disadvantageous if the vertical spring force exerted by such magnetic spring has to be led around the spring by mechanical means.

OBJECT OF THE INVENTION

It is an object of the invention to provide an improved vibration isolator.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, this object is achieved by providing a vibration isolator comprising a base structure, a load structure, a displacement structure and at least one vertical air gap formed by opposing and substantially parallel walls of the base structure and the load structure. The opposing walls are at least partly covered by respective arrays of permanent magnets, neighboring magnets in the arrays having alternating magnetization directions and an arrangement of the permanent magnets in the arrays being such that a gravitational force on the load structure is substantially compensated by a net magnetic force of the base structure on the load structure. The magnetization direction alternates at least in the vertical direction. The displacement structure is for relatively displacing at least one array of permanent magnets with respect to its opposing array of permanent magnets in a horizontal direction along the air gap for adjusting a load capacity of the vibration isolator.

A difference with the magnetic spring of Robertson et al. is that a vertical air gap is used instead of a horizontal one. In Robertson et al., the load structure is pulled towards or pushed away from the magnetic surfaces above or beneath the load structure. Thus, the magnetic force works perpendicular to the opposing magnetic surfaces. The permanent magnets are magnetized in a direction parallel to the direction of the force of gravity which is pulling down the load structure. According to the invention, the matrix arrangement of the permanent magnets is such that individual magnets in the base structure may attract or retract individual magnets in the load structure and vice versa, but the resulting net magnetic force causes the load structure to be pushed in an upward direction, i.e. against gravity. When the magnetic surfaces come in pairs any horizontal forces that may occur are canceled. The load structure is pushed upward in a direction parallel to the opposing surfaces of the magnetic arrays. The vertical air gap gives a high magnetic force with low stiffness. A big advantage of the vibration isolator according to the invention is that the net magnetic force in the vertical direction is constant over a larger range.

The displacement structure is coupled to one or more of the arrays of permanent magnets of the base structure and/or of the load structure. The displacement structure is capable of moving one or more arrays of permanent magnets on the base structure and/or one or more arrays of the permanent magnets on the load structure such that a vertical force generated by the vibration isolator in a direction opposite to the gravitational force changes. Thereby the vibration isolator can be optimized for different weights of the load structure and/or different weights of loads provided on the load structure.

It is to be noted that the vibration isolator does not only protect against vibrations, but also against other mechanical disturbances. However, the word vibration isolator will be used, because vibrations are the most common type of mechanical disturbances the system according to the invention tries to deal with.

In an embodiment of the vibration isolator according to the invention, one of the base structure and the load structure comprises a receiving volume and another one of the base structure and the load structure comprises a block. The shapes and dimensions of the receiving volume and the block are such that the block fits into the receiving volume in order to provide the vertical air gap. The respective arrays of permanent magnets are arranged at least at one inside wall of the receiving volume and at least one outer wall of the block.

In preferred embodiments, multiple walls of the receiving volume and of the load structure are used for carrying arrays of permanent magnets. As a result, possible horizontal components of the net magnetic force on one wall of the load structure may be compensated by horizontal components of the net magnetic force on other walls of the load structure. Alternatively, a non-flat, e.g. circular receiving volume may cause such compensation. In an embodiment, a horizontal cross-section of the receiving volume is cross shaped. If legs of the cross shaped receiving volume are equally distributed around a center of the cross shape and if the arrays of permanent magnets are arranged equally in all the legs, the net magnetic horizontal force may be zero.

In an embodiment, at least one of the arrays of permanent magnets provided on the inner surfaces of the receiving volume is arranged movable in a direction towards and away from a vertical centerline of the cross shaped receiving volume. Alternatively, or in addition, at least one of the arrays of permanent magnets provided on the outer surface of the block is arranged movable in a direction towards and away from the vertical centerline of the cross shaped receiving volume. It is not necessary to displace all arrays of permanent magnets. If already one array of permanent magnets is displaced, the vertical force generated by the vibration isolator changes and, thus, the load capacity of the vibration isolator changes. If only a single array is or a few arrays of permanent are displaced, care must be taken that the load structure does not become unbalanced—an unbalance of the load structure may also be corrected by placing the load at another location on the load structure. Also, if only a single array of permanent magnets is displaced with respect to its opposing array of permanent magnets, care must be taken that a resulting change in a net horizontal magnetic force does not displace the load structure more than allowed. In an embodiment, a plurality of arrays of permanent magnets are displaced with respect to their opposing arrays of permanent magnets such that a net change of the horizontal magnetic components applied to the load structure is about zero and such that changes of the vertically oriented magnetic forces are equally distributed over the load structure.

In an embodiment, the displacement structure comprises a guiding recess or a guiding protrusion for guiding and allowing a movement of respective arrays of permanent magnets. In an embodiment, the guiding recesses or the guiding protrusion guide and allow a movement of respective arrays of permanent magnets in the direction towards and away from the vertical centerline of the cross shaped receiving volume. Guiding recesses and guiding protrusions are effective means for controlling a positions of the arrays of permanent magnets in particular when the guiding recesses and guiding protrusions cooperate with an opposing protrusion or opposing recesses, respectively. The guiding recesses and guiding protrusions may have an elongated shape.

In an embodiment, the displacement structure comprises at least one linearly displacement structure for linearly displacing at least one of the arrays of permanent magnets of the opposing walls, the linear displacement structure is, for example, one of i) a rack and pinion, the rack being coupled to the at least one of the arrays of permanent magnets, ii) a leadscrew and a screw arranged around the leadscrew, the screw being coupled to the at least one of the arrays of permanent magnets. Such linearly displacing structures are effective means for displacing the at least one of the arrays of permanent magnets. Furthermore, such means translate a rotational movement into a linear movement and, as soon as no rotational movement is received, the linear location of the at least one of the arrays of permanent magnets often remains relatively stable. The vibration isolator may comprise at least one electromagnetic compensating unit for actively controlling relative movements of the load structure with respect to the base structure. The compensating units may be part of the base structure and/or the load structure. Such compensating units may fulfill two functions. First, they may bring the load structure back to a neutral position when the spatial deviation of the load structure from this neutral position tends to become too large. Second, they provide stabilization of the load structure, to improve the stability of the passive permanent magnet structure.

These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1a shows a platform supported by vibration isolators according to the invention,

FIG. 1b shows a close-up of a vibration isolator according to the invention,

FIG. 2a shows the load structure of the vibration isolator shown in FIG. 1 b,

FIG. 2b shows the base structure of the vibration isolator shown in FIG. 1 b,

FIG. 3a schematically shows how two opposing arrays of permanent magnets may cause an upward net magnetic force,

FIG. 3b shows a schematic representation of horizontal and vertical components of the net magnetic force as a function of vertical displacement of the load structure,

FIG. 4 shows an exemplary arrangement of permanent magnets in an array for use in the vibration isolator according to the invention,

FIG. 5a shows an alternative way of arranging two opposing arrays to cause an upward net magnetic force,

FIG. 5b shows two slightly tilted vertical air gaps,

FIG. 6a shows a horizontal cross section of a vibration isolator with a square shaped receiving volume,

FIG. 6b shows a horizontal cross section of a vibration isolator with a cross shaped receiving volume,

FIG. 7 shows a perspective view of a cross shaped load structure,

FIGS. 8a, 8b, 8c and 8d show possible topologies of a load structure according to the invention,

FIG. 9 shows in an exploded view an example of a vibration isolator,

FIG. 10a shows a cross-sectional view of an arrangement of opposing arrays of permanent magnets of the embodiment of FIG. 9 in which a plurality of arrays of permanent magnets are displaced with respect to their opposing arrays of permanent magnets,

FIG. 10b shows a cross-sectional view of another arrangement of opposing arrays of permanent magnets in which a plurality of arrays of permanent magnets are displaced with respect to their opposing arrays of permanent magnets,

FIG. 11 shows a three dimensional view of the base structure or load structure of an embodiment of a vibration isolator,

FIG. 12 shows a cross-sectional view of the structure of FIG. 11 along line XII-XII′.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1a shows a platform 14 supported by vibration isolators 10 according to the invention. The platform 14 carries a payload 15 which requires very accurate positioning and vibration isolation. In many technical areas like, e.g., lithographic industry, electron beam microscopy and space applications, it is very important to reduce the influence of vibrations on the accuracy of the positioning systems. The exemplary platform 14 is used in a lithographic machine and carries a lens system 15 for accurately focusing a laser bundle on a silicon wafer positioned underneath the platform 14. Vibration isolators 10 reduce the vibrations in this part of the machine and keep the lens system 15 aligned with the wafer.

FIG. 1b shows a close-up of a vibration isolator 10 according to the invention. The vibration isolator comprises a base structure 11 and a load structure 12. The load structure 12 fits into a receiving volume of the base structure 11, such that vertical airgaps 13 are formed in between the inner walls of the receiving volume and the opposing outer walls of the load structure 12. It is to be noted that the ‘airgaps’ 13 are not necessarily filled with air, but may also be filled with another gas or gas mixture. In a vacuum environment, the vertical airgaps 13 may just consist of empty space. The payload 15 may be positioned directly on the top surface of the load structure 12 or one to more vibration isolators 10 may be used for supporting a platform 14 for carrying the payload 15. It is to be noted that the respective functions of the base structure 11 and the load structure 12 may be interchanged. When turned upside down, the load structure 12 carries the base structure 11. The receiving volume of the base structure 11 then fits over the load structure 12 and carries the payload 15 or the platform. However, in the following, it will be assumed that the vibration isolator 10 is used in an orientation like shown in FIG. 1b and that the platform 14 is supported by the load structure 12. Similarly, it will be assumed that the load is placed on top of the vibration isolator 10. The vibration isolator 10 according to the invention may however also be used with the load hanging underneath the vibration isolator 10.

FIG. 2a shows the load structure 12 of the vibration isolator 10 shown in FIG. 1b . The load structure 12 is a massive or hollow block of some material, e.g. a non-magnetic metal like aluminum or a magnetic metal like iron. As will be elucidated later on, the weight of the load structure 12 should be such that the upward magnetic force of the vibration isolator 10 compensates for the gravity pulling at the load structure 12, platform 14 and payload 15. The outer walls of the load structure 12 are supplied with arrays of permanent magnets 22. Neighboring magnets in the arrays 22 have alternating magnetization directions. The magnetization direction at least alternate in the vertical direction. At least part of the magnets in the arrays 22 are magnetized in a direction perpendicular to the outer wall to which the array 22 is applied.

FIG. 2b shows the base structure 11 of the vibration isolator 10 shown in FIG. 1b . The base structure 11 is a box with, in this example, a rectangular shaped receiving volume. The shape and dimensions of the receiving volume are such that the base structure 11 is able to receive the load structure 12 in order to form the vertical airgaps 31 shown in FIG. 1b . The inner walls of the base structure 11 are also supplied with arrays of permanent magnets 21. Neighboring magnets in the arrays 21 have alternating magnetization directions. The magnetization directions alternate at least in the vertical direction. At least part of the magnets in the arrays 21 are magnetized in a direction perpendicular to the inner wall to which the array 21 is applied.

In the embodiment of FIGS. 2a and 2b , some space is left open between neighboring magnets in the array, but the magnets may also be arranged without such a gap.

When the load structure 12 is inserted into the receiving volume of the base structure (as shown in FIG. 1b ), the magnets in the opposing arrays 21, 22 of the base structure 11 and the load structure 12 cause a net magnetic force that compensates for the weight of the load structure 11, the platform 14 and the payload 15. When the total load weight of the load structure 11, the platform 14 and the payload 15 is in balance with this net magnetic force, the load structure 11 floats in a meta-stable position within the receiving volume, without making contact with any of the inner walls or the bottom of the receiving volume. When the base structure 12 moves relative to the load structure 11, e.g., caused by vibrations, the changing net magnetic force brings the load structure back to this stable position.

To prevent the opposing magnetic arrays 21, 22 from touching each other, mechanical stops may be added to the base structure 11 and/or the load structure 12.

FIG. 3a schematically shows how two opposing arrays 21, 22 of permanent magnets 31, 32 may cause an upward net magnetic force 33. FIG. 3a shows the base structure magnetic array 21 and part of the base structure 11 to which it is attached. The base structure magnetic array 21 comprises three horizontally magnetized permanent magnets. The upper and lower magnets 31 have a left to right magnetization. The middle magnet 32 has a right to left magnetization. The shown load structure magnetic array 22 only has two horizontally magnetized permanent magnets. The upper magnet 31 has a left to right magnetization. The lower magnet 32 has a right to left magnetization. A vertical airgap 13 is formed between the opposing arrays 21, 22.

The load structure magnetic array 22 is positioned such that opposing magnets 31 with a left to right polarization are vertically displaced approximately half the height of one magnet 31. Consequently, the upper magnet of the load structure magnetic array 22 is simultaneously attracted by the upper magnet of the base structure magnetic array 21 and repelled by the middle magnet of the base structure magnetic array 21. The horizontal components of these two magnetic forces cancel each other at least partially, while the vertical components add up. The net magnetic force upon the upper magnet of the load structure magnetic array 22 is therefore directed upwards. In a similar way, the lower magnet of the load structure magnetic array 22 is also pushed upward by the magnets of the base structure magnetic array 21. The net magnetic force on the load structure 12 has a relatively large vertical component 33 and, in this example, a small horizontal component. When the vertical position of the load structure magnetic array 22 changes, also the vertical and horizontal components 33, 34 of the net magnetic force change.

FIG. 3b shows a schematic representation of horizontal and vertical components 34, 33 of the net magnetic force as a function of vertical displacement of the load structure 12. This diagram shows how the vertical and horizontal components 33, 34 of the net magnetic force vary with the vertical displacement of the load structure 12. In the middle of the diagram, where the x-axis and the y-axis cross, the vertical displacement is zero. This represents the situation in which the load structure is positioned such that opposing magnets with equal polarization are vertically displaced exactly half the height of one magnet. In this situation there is no net magnetic force in the horizontal direction 34 and the vertical component 33 of the net magnetic force is at its maximum. If the vertical gravity force acting on the load structure is equal and opposite to the vertical magnetic force, the system is in a metastable equilibrium. The positive x-axis represents upward vertical displacement. The negative x-axis represents downward vertical displacement. The y-axis represents the magnitude and direction of the horizontal and vertical components 34, 33 of the net magnetic force.

Upward vertical displacement of the load structure 12 results in a slightly decreasing vertical force component and an increasing horizontal component 34 in the right to left (negative) direction. Downward vertical displacement results in a slightly decreasing vertical force component and an increasing horizontal component 34 in the left to right (positive) direction. Such horizontal components 34 can be compensated by further vertical airgaps 13 at other walls of the vibration isolator 10 (see e.g. FIG. 1b ) and/or by active electromagnetic actuators. The dashed line 35 shows the vertical displacement and corresponding net magnetic force components 33, 34 for the situation of FIG. 3 a.

Horizontal displacement to the right (airgap enlarged) reduces both the horizontal and vertical force component. Horizontal displacements that reduce the airgap size increase both the vertical and horizontal force component.

The big advantage of using the vertical airgaps 13 according to the invention is that the vertical component 33 of the net magnetic force does not vary much when the load structure 12 is vertically displaced. The use of additional airgaps helps minimizing the instabilities when the device is not operated in its equilibrium point, especially because the horizontal force components cancel or at least reduce each other. The vibration isolator 10 thus obtained has a low stiffness over the full range of movement.

FIG. 4 shows an exemplary arrangement of permanent magnets 41, 42 in an array 21, 22 for use in the vibration isolator 10 according to the invention. Thirteen permanent magnets 41 in the 5-by-5 array have a magnetization direction pointing into the surface shown in the figure. The remaining twelve permanent magnets 42 in the 5-by-5 array have a magnetization direction pointing into the surface shown in the figure. Arrays having other dimensions may also be used. The smallest possible array to use is a 1-by-2 array. It is also possible to use patterns, wherein different magnets have different dimensions or wherein more than two different magnetization directions are used. Part of the array may be ‘empty’ or filled by non-magnetized material.

FIG. 5a shows an alternative way of arranging two opposing arrays 54, 55 to cause an upward net magnetic force. Still, most of the permanent magnets 31, 32 are magnetized in a horizontal direction. Some smaller permanent magnets 53, in between the horizontally magnetized ones, are magnetized in a vertical direction. The configuration thus obtained is a Halbach or quasi-Halbach configuration. Such configurations are known for the property to focus the magnetic field to one side of the magnets only, in this case the side of the airgap.

FIG. 5b shows two slightly tilted vertical air gaps 13. In this example, the base structure 51 is situated in a receiving volume of the load structure. The walls of the base structure and the load structure are slightly tilted with respect to the direction of gravity. The horizontal components of the net magnetic forces on the load structure magnetic arrays 52 cancel each other. The load structure is pushed upward by the net magnetic force and pulled down by gravity. When both horizontal force components cancel each other, the load structure is in a metastable position. The shown arrangement of the magnetization directions in the different arrays of magnets is just one of the many possible arrangements that may result in this metastable position.

FIG. 6a shows a horizontal cross section of a vibration isolator 10 with a square shaped receiving volume. Here we assume that the receiving volume is part of the base structure 11, but when the vibration isolator 10 is turned upside down the receiving volume is part of the load structure 12. The opposing magnetic arrays 21, 22 form four vertical gaps 13 with different orientations. The magnetization of the permanent magnets in the arrays 21, 22 is preferably such that all horizontal components of the net magnetic force cancel each other almost fully. A disadvantage of the arrangement shown in this figure is that horizontal displacement of the load structure 12 results in a relatively large torque on this load structure 12. The magnetic array 22 which is closest to a magnetic array 21 of the base structure 11 will be pushed upward with a greater force than the magnetic array 22 in the wider vertical airgap 13. The total upward magnetic force on the load structure 12 will be the same, but the load structure will tend to rotate around the horizontal axis perpendicular to the direction of horizontal movement.

FIG. 6b shows a horizontal cross section of a vibration isolator 60 with a cross shaped receiving volume. An advantage of this configuration is that a very low torque is obtained, which will improve the stability. For further improving the stability of the vibration isolator 60, active electromagnetic actuators 65 are included in the load structure 61 and/or the base structure 63. When the load structure 61 tends to depart from its stable position too much or when it starts rotating, the active electromagnetic actuators 65 can be used for bringing it back to the desired position and orientation. The electromagnetic actuators 65 may also be used when assembling the vibration isolator 60 and inserting the base structure 63 into the receiving volume for bringing it in the right position with respect to the load structure 61.

FIG. 7 shows a perspective view of a cross shaped base structure 63. The base structure 63 comprises a support block 71 for mounting the cross shaped array carrier 73. Arrays of permanent magnets 64 are applied to the surfaces of the cross shaped array carrier 73. In this example, four rows of magnets are provided. For example, the first and third row have a magnetization direction pointing into the array carrier 73 and the second and fourth row have a magnetization direction pointing out of the array carrier 73. Also checker board patterns and other alternating patterns may be used. In the checker board pattern the magnetization direction alternates between neighboring magnets in the vertical and the horizontal direction. In an embodiment, if the arrays of permanent magnets have a checker board pattern, then the displacement of the opposing arrays is performed in discrete steps such that, in the horizontal direction, the displaced array(s) moves a length of two permanent magnets, or four permanent magnets, etc. Thereby the horizontal force decreases. In another embodiment, if the arrays of permanent magnets have a checker board pattern, then the displacement is performed on a continuous scale and not in discrete steps. In this another embodiment, initially, an increase in a horizontal force occurs while at the same time the vertical force reduces. Subsequently, if the displacement is larger than the length of one permanent magnet and is going towards a length of two permanent magnets, the horizontal force decreases and the vertical force increases. Thus, a small displacement of the arrays of permanent magnets having a checker board pattern can also be used for correcting for a slightly higher load. In this embodiment it is assumed that the reduction of the vertical force does not results in a too large unbalance of the load structure, or two or more arrays of permanent magnets are displaced simultaneously to obtain a net change of the vertical force received by the load structure that is about zero.

In this embodiment, active electromagnetic actuators with the coils 65 are applied to the support block 71 of the base structure 63. They interact with permanent magnets on the load structure 61 for actively controlling the position of the load structure 61 relative to the base structure 63 when the passive control by the permanent magnet arrays 64 is not sufficient. For this active control, the coils 65 may interact with the magnets of the permanent magnet arrays 64 of the load structure or with separate and dedicated control magnets (not shown) closer to the coils. Cooling ducts 72 may be provided in, e.g., the support block 71 for minimizing temperature fluctuations in the vibration isolator. Of course, it is also possible to put the coils 65 in the load structure 61 and let them interact with magnets in the base structure 63.

Instead of or in addition to the electromagnetic actuators, mechanical stops, springs or the like may be used for improving the stability of the vibration isolator and for preventing contact between the permanent magnets of the base structure 63 and the load structure 61.

A removable end stop 74 prevents the load structure 61 from being separated from the base structure 63 when the net magnetic force exceeds the gravity force, e.g. when the payload 15 is taken from the platform 14. When assembling the vibration isolator, the end stop 74 is removed and the cross shaped array carrier 61 is inserted into corresponding slots (=the receiving volume) of the base structure. From the other side of the base structure, the end stop 74 is then applied to the load structure 63 again. The load structure 63 can only be separated from the base structure if the end stop 74 is removed first. FIGS. 8a, 8b, 8c and 8d show four possible topologies of a base or load structure according to the invention. FIG. 8a shows a cross shape like already shown in FIGS. 6b and 7. FIG. 8b shows a cross with six arms and FIG. 8c shows a cross with 8 arms. FIG. 8d show a three-arms cross. In order to obtain an embodiment with low torque on the load structure, it is preferable to use thin arms. It is to be noted that cross-shaped topologies may have three or more arms equally distributed along a central axis.

FIG. 9 shows in an exploded view an example of a vibration isolator. In particular, a portion of the load structure 101 and a portion of the base structure 102 is shown. In line with previous discussions, it is to be noted that element 102 may also be the load structure and element 101 may also be the base structure. The load structure 101 and the base structure 102 have arrays of permanent magnets with, in the vertical direction, alternating magnetization directions—for example, a (horizontal) row 103 of permanent magnets of a first magnetization direction and that is interposed between two (horizontal) rows 104 of a second magnetization direction (opposite to the second magnetization direction). If the load structure 101 is arranged around the base structure 102, as shown in previous figures and discussed previously, the load structure 101 receives and upwards oriented resulting magnetic force that compensates the gravity force received by the load structure 101. The amount of compensation of the gravity force strongly depend on the force that the opposing magnets provide to each other—this depends on, among other factors, the strength of the magnets, the area of opposing arrays of permanent magnets and the depth of the vertical air gap. If, subsequently, the load changes, it may be that the resulting magnetic force received by the load structure 101 becomes too large. The displacement structure provides a solution for preventing that the (vertically oriented) resulting magnetic force received by the load structure 101 is too large or too small. By displacing one or more of the arrays of permanent magnets in the horizontal direction, the areas of the arrays of permanent magnets that are directly opposite to each other changes and thereby the (vertically oriented) resulting magnetic force received by the load structure 101 changes as well. If the displacing is such that the area of permanents that are arranged directly opposite to each other increases, then (the vertically oriented) resulting magnetic force received by the load structure 101 increases as well, and vice versa.

FIG. 10a shows a cross sectional view along a horizontal plane of an arrangement of opposing arrays of permanent magnets 201, 202 of the embodiment of FIG. 9 in which a plurality of arrays of permanent magnets are displaced with respect to their opposing arrays of permanent magnets. In FIG. 10a the arrays of permanent magnets 202 of the load structure are hatched and the arrays of permanent magnets 201 are the arrays of the base structure. If it is assumed that directly after manufacturing or assembly the vibration isolator the array permanent magnets 201, 202 are directly opposite to each other, then FIG. 10a shows a situation wherein either the arrays of the permanent magnets of the load structure 202 or the arrays of the permanent magnets 201 of the base structure are moved into a direction towards a center of the cross-shaped load structure or a direction away of the center of the cross-shaped load structure. Also, in an embodiment, the displacement structure may displace the arrays 201, 202 of both the load structure and the base structure. If the arrays 201, 202 of both the load structure and the base structure are moved, two opposing arrays move in opposing directions.

FIG. 10b shows a cross-sectional view along a horizontal plane of another arrangement of opposing arrays of permanent magnets 251, 252 in which a plurality of arrays of permanent magnets are displaced with respect to their opposing arrays of permanent magnets. The shown cross-sectional view is, for example, a cross sectional view of the arrays of permanent magnets of the embodiment of FIG. 1b . It is seen in FIG. 10b that the arrays of permanent magnets 251, 252 are not completely opposing to each other. In comparison to the situation of FIG. 1b , the arrays 252 of permanent magnets of the base structure 11 of FIG. 11 are displaced in a direction along the vertical air gap between the arrays of permanent magnets 251, 252.

FIG. 11 shows a three dimensional view of the base structure 300 of an embodiment of a vibration isolator. It is to be noted that the structure 300 may also be a load structure if the structure is arranged up-side-down and if an appropriate base structure is provided. An embodiment of a displacement structure is presented in FIG. 11. The displacement structure comprises elongated guiding recesses 310. The arrays of permanent magnets 302 are provided on walls that are each arranged in one of the elongated guiding recess 310. In the example of FIG. 11 a complete lower edge of the walls are arranged in the elongated guiding recess 310—alternatively the walls with the arrays of permanent magnets 302 have protrusions or wheels that extend into the elongated guiding recess 310. It is noted a top edge of the walls may also be arranged in an elongated guiding recess. Alternatively, the walls have one or more elongated guiding recessed and the housing of the base structure 300 has protrusions or wheels that cooperate with the elongated guiding recesses for allowing a movement of the walls with arrays of permanent magnets 302.

FIG. 12 shows a cross-sectional view of the structure 300 of FIG. 11 along a vertical plane that intersects with line XII-XII′. Reference number 302′ points towards a cross-sectional view of one of the walls on which arrays of permanent magnets 302 are provided. Edges 325 of the walls 302′ are received by the elongated recesses 310. The displacement structure comprises a leadscrew and screw arrangement 323 for each moveable wall. A rotatable axis 322 is provided and at a portion of its surface a thread is provided. The rotatable axis 322 is arranged in a hole through the wall 302′. The surface of the hole has also a thread that interacts with the thread of the rotatable axis 322 such that, if the rotatable axis 322 rotates, the wall 302′moves towards or away from the centerline of the cross-shaped wall structure. The presented displacement structure also comprises a gears arrangement 324 near the centerline of the cross-shaped wall structure for transferring a rotation of a central rotatable axis 321 towards a rotation of the rotatable axes 322 of the walls 302′. As shown in FIG. 12, the rotatable axes 322 may have at its end a gear which interacts with a gear of the central rotatable axis 321. In use, an actuator may rotate the central axis 321 resulting in a displacement of the walls 302′. In use, a human may rotate the central axis 321 for obtaining a specific displacement of the walls 302′.

It is to be noted that embodiments of the displacement structure are not limited displacement structures comprising the above described elongated guiding recess, the above described leadscrew and screw arrangement 323 and the above described gears arrangement 324. In the context of this document, each displacement structure that enables/allows the displacement of displacing at least one array of permanent magnets with respect to its opposing array of permanent magnets in a horizontal direction along the air gap for adjusting a load capacity of the vibration isolator may be used. For example, the leadscrew and screw arrangement 323 may be replaced by a rack and pinion structure wherein a rotation of the pinion results in a linear movement of the rack. In such an embodiment, appropriate means may be provided for rotating the pinion.

Further embodiment are defined in the subsequent clauses:

1. A vibration isolator comprising:

a base structure,

a load structure,

at least one vertical air gap formed by opposing and substantially parallel walls of the base structure and the load structure,

the opposing walls being at least partly covered by respective arrays of permanent magnets, neighboring magnets in the arrays having alternating magnetization directions, an arrangement of the permanent magnets in the arrays being such that a gravitational force on the load structure is substantially compensated by a net magnetic force of the base structure on the load structure.

2. A vibration isolator as claimed in clause 1, wherein the base structure is a box with a receiving volume and the load structure comprises a block, the shapes and dimensions of the receiving volume and the block being such that the block fits into the receiving volume in order to provide the vertical air gap, the respective arrays of permanent magnets being arranged at at least one inside wall of the receiving volume and at least one outer wall of the block. 3. A vibration isolator as claimed in clause 1, wherein the load structure is a box with a receiving volume and the base structure comprises a block, the shapes and dimensions of the receiving volume and the block being such that the block fits into the receiving volume in order to provide the vertical air gap, the respective arrays of permanent magnets being arranged at at least one inside wall of the receiving volume and at least one outer wall of the block. 4. A vibration isolator as claimed in clause 2 or 3, wherein a horizontal cross section of the receiving volume has a rectangular, triangular or circular shape. 5. A vibration isolator as claimed in clause 2 or 3, wherein a horizontal cross section of the receiving volume is cross shaped. 6. A vibration isolator as claimed in clause 1, wherein the base structure and/or the load structure comprise at least one electromagnetic compensating unit for actively controlling relative movements of the load structure with respect to the base structure. 7. A vibration isolator as claimed in clause 1, wherein the permanent magnets with the alternating magnetization directions form a checkerboard pattern. 8. A vibration isolator as claimed in clause 1, wherein the permanent magnets with the alternating magnetization directions form a Halbach or quasi-Halbach configuration. 9. A vibration isolator as claimed in clause 1, wherein the vertical air gap is slightly tilted with respect to a vertical axis.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb “comprise” and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article “a” or “an” preceding an element does not exclude the presence of a plurality of such elements. The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. 

1. A vibration isolator comprising: a base structure, a load structure, at least one vertical air gap formed by opposing and substantially parallel walls of the base structure and the load structure, the opposing walls being at least partly covered by respective arrays of permanent magnets, neighboring magnets in the arrays having alternating magnetization directions, an arrangement of the permanent magnets in the arrays being such that a gravitational force on the load structure is substantially compensated by a net magnetic force of the base structure on the load structure, wherein the magnetization direction alternates at least in the vertical direction, a displacement structure for relatively displacing at least one array of permanent magnets with respect to its opposing array of permanent magnets in a horizontal direction along the air gap for adjusting a load capacity of the vibration isolator.
 2. A vibration isolator as claimed in claim 1, wherein one of the base structure and the load structure comprises a receiving volume and the other one of the base structure and the load structure comprises a block, the shapes and dimensions of the receiving volume and the block being such that the block fits into the receiving volume in order to provide the vertical air gap, the respective arrays of permanent magnets being arranged at at least one inside wall of the receiving volume and at least one outer wall of the block.
 3. A vibration isolator as claimed in claim 1, wherein the load structure is a box with a receiving volume and the base structure comprises a block, the shapes and dimensions of the receiving volume and the block being such that the block fits into the receiving volume in order to provide the vertical air gap, the respective arrays of permanent magnets being arranged at at least one inside wall of the receiving volume and at least one outer wall of the block.
 4. A vibration isolator as claimed in claim 2, wherein a horizontal cross section of the receiving volume has a rectangular or triangular shape.
 5. A vibration isolator as claimed in claim 2, wherein a horizontal cross section of the receiving volume is cross shaped.
 6. A vibration isolator as claimed in claim 5, wherein at least one of: at least one of the arrays of permanent magnets provided on inner surfaces of the receiving volume is arranged movable in a direction towards and away from a vertical centerline of the cross shaped receiving volume, at least one of the arrays of permanent magnets provided on an outer surface of the block is arranged movable in a direction towards and away from the vertical centerline of the cross shaped receiving volume.
 7. A vibration isolator as claimed in claim 1, wherein the base structure and/or the load structure comprise at least one electromagnetic compensating unit for actively controlling relative movements of the load structure with respect to the base structure.
 8. A vibration isolator as claimed in claim 1, wherein the permanent magnets with the alternating magnetization directions form a checkerboard pattern.
 9. A vibration isolator as claimed in claim 1, wherein the permanent magnets with the alternating magnetization directions form a Halbach or quasi-Halbach configuration.
 10. A vibration isolator as claimed in claim 1, wherein the vertical air gap is slightly tilted with respect to a vertical axis. 